A new poll shows a surprisingly close contest in the special election to succeed former Rep. Anthony Weiner, with the Republican candidate trailing by just six percentage points in a heavily Democratic district.

The poll, released Wednesday by the Siena Research Institute, showed Democratic candidate David Weprin holding a 48%-42% lead over Republican Robert Turner.

The numbers are surprising because Democrats hold such a large registration advantage — better than 3-to-1 — in the Queens and Brooklyn district. The national political parties and outside spending groups have yet to show any significant investments in the race.

Siena’s pollster warned that numbers could change that. “This special election is a wide open race,” said pollster Steven Greenberg.

The election will be held on Sept. 13, the same day as the state’s primary. The congressional seat became open when Weiner resigned in June after admitting he exchanged sexually charged messages with about six women her met online.

There are no other major contests to draw voters to polls, so turnout is expected to be very light. That means organizational strength may play an important role in determining the winner.

Weprin, currently serving in the state Assembly, has sought to cast his rival as part of a national GOP effort to cut Medicare. Turner, a businessman, has tried to appeal to the orthodox Jewish voters in the district by arguing he, not Weprin, would do more to support Israel.

Political operatives suspect whoever wins the seat may not hold it for long. New York will lose two congressional seats in legislative redistricting to be completed before the 2012 election, and Weiner’s former district is widely expected to be carved out of existence.